1. Field of the Invention
The present invention relates to threaded connections employed to secure together the ends of relatively stiff, heavy tubular pipe members employed to drill through earthen formations. More particularly, the present invention relates to a thread for use in the box connection of drill collars, heavyweight drill pipe, or other relatively stiff tubular components of a drill stem assembly.
2. Setting of the Invention
Heavy, relatively stiff tubular bodies are employed at the bottom of the drill string primarily to apply weight to the drill bit and to maintain the drill string in tension. These bodies include drill collars and transition links called heavyweight drill pipe. The drill collars and heavyweight drill pipe differ from the standard drill string pipe in that bending stresses tend to concentrate in the connections of the drill collars and heavyweight drill pipe more so than in the tool joint connections of the more flexible drill pipe.
Cyclical bending during drilling induces fatigue failure in the tool joint and drill collar connections of the drill string assembly. Fatigue induced failure in the drill pipe normally occurs in the threaded male, or xe2x80x9cpinxe2x80x9d end of the connection, typically about two threads away from the shoulder or base of the pin thread. Fatigue induced failure of the threaded female or xe2x80x9cboxxe2x80x9d end of a drill pipe tool joint is not typical. The failure in drill collars or heavyweight drill pipe connections typically occurs in the box, several threads from the box base or shoulder. Pin failure in this type connection is not typical.
An explanation of the cause of fatigue failure in drill collar connections is set forth in United States Statutory Invention Registration No. H1329, published Jul. 5, 1994. The Registration describes a number of changes in the construction of a drill collar connection to reduce box connection failure. One such change is to provide an enlarged thread root in the box thread in the area where most failures occur. The root radius is a single radius described as being between 0.0500 inch and 0.0625 inch. The root radius of a standard API (American Petroleum Institute) thread form is 0.038 inch. A disadvantage of a large, singe radius root is that a significant part of the thread flank is lost.
An improved thread form for the pin connection of drill pipe tool joint described in U.S. Pat. No. 4,549,754 (the xe2x80x9c""754 patentxe2x80x9d), incorporated herein by reference for all purposes, is assigned to the assignee of the present application. The patented thread form makes several changes in a conventional API thread to achieve the reduction in damaging stress to the pin end of the pin and box connection in a drill pipe tool joint. First, the modified thread on the pin is made capable of mating with a standard API box thread so that the modified pin thread is compatible with the box threads on equipment already present in the field. Next, the radius of the pin threads is modified to relieve the stress at the root of the threads. Third, the thread taper is modified to create an artificial pitch difference between the pin and box member and to provide for a more even distribution of loading of the thread when fully made up in a box. Forth, the new geometry of the modified connection creates a radial interference starting from the threads remote from the pin shoulder to prevent over-torquing the connection.
The thread form of the ""754 patent is effective in reducing failure of the pin in the area of the base of the tool joint pin threads. The thread form is required only on the pin since box failures are rare in tool joint connections. The desired reduction in tool joint failure is thus fully attained by modifying only the pin of the connection. No significant benefit is achieved by providing the box of a tool joint with the modified thread form used on the pin and use of such thread form on the tool joint box would unnecessarily increase the cost of construction of the connection. Moreover, a connection in which the thread form was used in both the pin and the box would produce a connection in which the contact area of the stab flanks of the threads is reduced to the point of increasing the likelihood of thread gauling.
Unexpected, improved performance has been realized in the connections of drill collars and heavyweight drill pipe equipped with a modified box thread form that mirrors the thread form employed in the tool joint pin design of the ""754 patent. Unlike the invention of the ""754 patent, the thread form is employed in the box of the connection rather than the pin. The root of the box thread is provided with an enlarged radius and a planar stab flank surface that extends from the root toward the pitch diameter of the thread. Unlike the ""754 connection, the thread taper of the drill collar connection is left unchanged in both the pin and box. The box may be mated with a conventional pin thread and may be gauged with conventional box taper, pitch diameter, and thread height gauges. A drill collar connection made with a conventional API pin and mated with the box of the present invention exhibits unexpected and significantly improved resistance to fatigue-induced box damage as compared with a conventional API pin and box drill collar connection.
From the foregoing, it will be appreciated that a primary object of the present invention is to provide a fatigue-resistant box connection for a drill collar or heavyweight drill pipe, or other stiff body tubular drill string component.
Yet another object of the present invention is to provide an improved, fatigue-resistant drill collar box connection that can be employed with a conventional, API drill collar pin connection.
It is also an important object of the present invention to provide an improved, fatigue-resistant drill collar box connection for use in a drill string assembly wherein the box connection may be gauged for taper, thread height, pitch diameter, and thread lead using conventional thread gauges.
The foregoing, as well as other, objects, features, and advantages of the present invention will be more fully appreciated and understood by reference to the following drawings, specification, and claims.